Project summary Spontaneous age-associated mutations in blood cells of older people are newly discovered risk factors for cardiovascular disease (CVD) and various blood cancers. CVD remains the leading cause of death among older people. The biological mechanisms of how these mutant blood cells escape the immune system would be of great interest in light of the overall success with immune checkpoint inhibitors in cancer treatment and in other human diseases. T cells are part of the immune system and express proteins that signal tolerance to healthy cells of the body (self). In the case of cancerous cells, ideally these signals should be turned off (or can be manipulated as such) so that the immune system can eliminate these cells. The idea of eliminating pre-cancerous blood cells associated with detrimental health outcomes may have wide-spread impact in the aging population. We chose to focus on mutations in TET2, a gene involved in regulating how active genes are, because it is one of the most common mutations found in blood cells of older people. The overall goal of the project is to identify immune checkpoint abnormalities in human T cells and monocytes harboring pathogenic TET2 mutations, which will not only reveal important biomarkers, but should provide mechanistic insights that will inform therapeutic strategies. We will use single-cell RNA sequencing, a sophisticated technology that will isolateTET2 mutation-driven effects on the cells' function. We will also treat T cells with immune checkpoint inhibitors and expose the cells to mutant monocytes to measure whether this treatment enhances T cell-mediated killing of mutant monocytes. The findings from these studies are expected to generate new hypotheses and support the concept of a clinical study aimed at evaluating the role of immune checkpoint inhibition as a therapeutic strategy in clonal hematopoiesis. An effective therapeutic strategy may reduce CVD-related morbidity and deaths in older people.